Rhexinema, Geitler, 1942

Darienko, Tatyana & Pröschold, Thomas, 2017, Toward a monograph of non-marine Ulvophyceae using an integrative approach (Molecular phylogeny and systematics of terrestrial Ulvophyceae II.), Phytotaxa 324 (1), pp. 1-41 : 26

Rhexinema versus Pleurastrum and Gloeotilopsis

The genus Rhexinema was described by Geitler (1942) to separate Pleurastrum paucicellulare from other species of this genus based on the following morphological criteria: very short branched filaments that easily disintegrate, parietal chloroplasts with a pyrenoid, and biflagellated zoospores. P. paucicellulare was originally described by Vischer (1933) from neustons in stagnant water near Basel, Switzerland (authentic strain SAG 463-1). Tupa (1974) did not support Rhexinema and left this species with Pleurastrum , in contrast to Lukešová (1991) and Ettl & Gärtner (1995), who accepted the genus as described by Geitler.

Friedl (1996) used phylogenetic analyses of SSU rDNA sequences to determine the polyphyly of the genus Pleurastrum and demonstrated the close relationship of Pleurastrum paucicellulare (SAG 463-1) with Protoderma sarcinoidea (UTEX 1710) and Gloeotilopsis planctonica (SAG 29.93). On this basis, he proposed the new combinations of Gloeotilopsis paucicellulare and G. sarcinoidea . Unfortunately, as demonstrated in our study, the identification of the strain SAG 29.93 was incorrect and did not fit with the original description by Iyengar & Philipose (1956). Therefore, all other combinations with Gloeotilopsis must be changed. Despite the different opinions about the genus Rhexinema by Tupa (1974), Geitler’s proposal was based on the authentic material of Vischer for Pleurastrum paucicellulare . Therefore, the genus Rhexinema has been validly described, and we proposed the following new combinations and species. Interestingly, several strains assigned as species of other genera also belong to Rhexinema . The authentic strain of Pseudendoclonium basiliense var. brandii and a strain identified as Helicodictyon planctonica are also members of Rhexinema . Both strains exhibit a morphology similar to their original descriptions by Vischer (1933) and Whitford & Schumacher (1966), as demonstrated in our study.